1. Field of the Invention
The invention relates to a coordinate measurement apparatus and to a method for operating a coordinate measurement apparatus, according to the preamble of the respective independent patent claims.
2. Description of Related Art
Coordinate measurement apparatus, often called laser trackers, are applied for the measurement of the position moving target points. The term laser trackers is to be understood as devices which comprise at least one distance meter operating with a focused laser beam (indicated as measurement beam in the following description). For example, the direction of the measurement beam is set to the target point with the help of a mirror rotatable about two axes, and is detected with angle sensors assigned to the rotation axes. The target point to be measured is provided with a retroreflector (in particular cube-corner prism or arrangement of three mirrors which are perpendicular to one another), wherein the retroreflector reflects the measurement beam of the laser tracker which is incident thereon, back to this laser tracker. Thereby, the reflected measurement beam runs coaxially to the emitted measurement beam if the measurement beam is incident on the reflector in an exactly centric manner, and offset parallel thereto, if the measurement beam is not centrically incident on the reflector. One concludes an absolute distance between the laser tracker and the target point and/or a change of this distance from a comparison of the emitted and reflected laser light, depending on the embodiment of the tracker (absolute distance meter or interferometer). The position of the reflector or of the target point relative to the tracker is computed from the angles detected by the angle sensors and from the distance detected by the distance meter.
A part of the reflected measurement beam is usually guided onto a PSD (position sensitive device). One may deduce the parallel shifting of the reflected measurement beam relative to the emitted measurement beam from the position, in which the reflected measurement beam is incident on the light-sensitive surface of the PSD. The measurement data which is determined by way of this defines the parallel offset of the reflected measurement beam and is used for a control of the measurement beam direction, in a manner such that the measurement beam follows the target point (tracking), when this moves. This means that by way of a suitable change of the measurement beam direction or of the alignment of the mirror aligning the measurement beam, one ensures that the parallel offset between the emitted and the reflected measurement beam is reduced or remains as small as possible.
Various arrangements of optical distance meters and picture detection sensors are described in WO 2007/079600 A1. In one arrangement (FIG. 4), an overview camera (104), a position-sensitive diode (109) operating in the visible spectral range and a zoom camera (106) are all coupled into the measurement light path of a distance measurement device (200, 300). The position-sensitive diode (109) must operate in the visible range, in order to be able to also use a precise measurement light beam of a He-Ne laser of an interferometer. A reflector illumination (110) is arranged outside the measurement light path and operates in the visible range in a manner matched to the overview camera, for detecting reflected targets. The overview camera must operate in the visible range, in order to be able to produce pictures of a good quality, in particular colour pictures.
U.S. Pat. No. 6,504,602 B1 describes a theolodite with a distance measurement apparatus. An automatic target recognition uses radiated light in the visible range or in the near infrared range.
EP 2 071 283 A2 describes the use of two separate cameras with wide and narrow viewing angles, in each case with their own light source coupled into the camera optical system. The cameras are arranged separately from one another, one of which with the viewing axis collinear to a distance meter, and operate with visible light. A target recognition is accomplished in each case by way of switching the respective light source on/off and a subsequent difference formation from the respective pictures.